Phnom Penh: The US President, Barack Obama, has denied his historic visit to Burma is an endorsement of the country's military-dominated government, saying it is instead an acknowledgement of reforms under way in the impoverished country.

"We understand it's a work in progress," Mr Obama said in Bangkok hours before he was due to fly to Burma's largest city Rangoon on Monday, becoming the first sitting US president to go there.

I don't think anyone is under the illusion that Burma has arrived.

"The country has a long way to go. I'm not somebody who thinks that the United States should stand on the sidelines and not want to get its hands dirty when there's an opportunity for us to encourage the better impulses inside a country," he said.

Mission to Asia ... Barack Obama tours the reclining Buddha in Bangkok before his historic trip to Burma. Photo: Reuters

Mr Obama said Burma's President, Thein Sein, a former general, is "taking steps that move us in a better direction". "But I don't think anyone is under the illusion that Burma has arrived," he said.

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Human Rights groups say Mr Obama should not be going to Burma until reforms have been consolidated and the government has done more to stop ethnic violence against Muslim Rohingya in the country's western Rakhine state.

More than 200 political prisoners remain in custody and the government continues to wage war against insurgents in Kachin state.

Speaking at a news conference, Mr Obama said it was "no accident" his first foreign trip after winning re-election was to Asia. The US was a "Pacific nation" and the Asia-Pacific region would be crucial for creating jobs in the US and shaping its security and prosperity.

But Mr Obama disembarked in Bangkok's steamy heat still thinking about problems at home.

Touring the sprawling temples and gardens of Wat Pho Royal Monastery in bare feet, Mr Obama was overheard telling a monk in bright orange robes that "we've been working on this budget. We're going to need a lot of prayer for that," a reference to the "fiscal cliff" negotiations in Washington.

Accompanied by the Secretary of State, Hillary Clinton, Mr Obama met Thailand's 84 year-old King Bhumibol Adulyadej, who has been in hospital since 2009.

"It's a great honour," Mr Obama said, grasping King Bhumibol's hand.

Analysts say Mr Obama's three-day whirlwind visits to Thailand, Burma and Cambodia aim to reinforce his administration's commitment to deepen economic and security ties to the region to counter China's rising power and influence.

Mr Obama will fly to Cambodia late on Monday for the East Asian Summit, a forum of 18 world leaders, where the 10 members of the Association of South-East Asian Nations have shown a rare display of unity against China's sweeping claims to the South China Sea, calling for the first formal talks with Beijing over the dispute that has raised tensions and exposed deep divisions in the region.

The Chinese premier, Wen Jiabao, has arrived in the Cambodian capital, Phnom Penh, for formal talks with ASEAN leaders, who agreed on Sunday to tell him they want to begin talks on a binding code of conduct aimed at reducing tensions in the South China Sea.

"They [ASEAN] would like to see the commencement of the discussion as soon as possible because this is an issue of interest, concern and worry of the international community," Mr Pitsuwan said.

Asked about ASEAN's decision, China's foreign ministry spokesman, Qin Gang, said consultations with ASEAN nations were continuing and the issue should not be a "stumbling block" for relations between China and the region.

China has resisted multilateral talks on the competing claims to the sea, preferring to try to negotiate disputes with each of the far less powerful claimants.